The proposed research involves three objectives: 1) To study the mechanism of action of chemotherapeutic agents in intact cells; 2) to evaluate the molecular mechanisms responsible for the occurrence of metabolic synergism; and 3) to determine the role of salvage pathways in modifying the chemotherapeutic effectiveness of anticancer agents. These objectives will be approached through the use of three primary techniques: 1) The development of a cell culture system designed to evaluate the chemotherapeutic interaction between pairs of metabolic inhibitors; 2) a determination of the concentrations of the key regulatory effectors of DNA synthesis under conditions of variable steady-state growth; and 3) the use of a system designed for the continuous infusion into mice. Of particular interest will be studies aimed at evaluating the functional metabolic regulation involved in controlling the supply of precursors of DNA synthesis through the further development of a mathematical model of this system. These approaches should lead to a more rational use of antimetabolites, both alone and in combination, for cancer chemotherapy. BIBLIOGRAPHIC REFERENCES: Semon, J., Grindey, G., Orsini, F., and Mihich, E., Separation of Anticancer Activity and Host Toxicity. Proc. WHO Conference, Milan, Italy, November 1975. In: Rational Design and Synthesis of Compounds for Cancer Chemotherapy, 1976, p. 46. Semon, J. H. and Grindey, G. B., Effect of Thymidine on the Therapeutic Selectivity of Methotrexate in Mice. Proc. Am. Assoc. Cancer Res. 17: 82, 1976.